1. Field of the Invention
The present invention relates to an optical data recording and reproducing apparatus which optically writes data onto or reads data from an optical disc. More precisely, it relates to a driving apparatus which electromagnetically drives an objective lens thereof.
2. Description of Related Art
In a known optical disc apparatus, such as an opto-magnetic disc apparatus, an objective lens is held on a carriage which is movable in a radial direction (i.e., tracking direction) of the optical disc, and the objective lens is movable in the tracking direction and the focusing direction through an elastic member. To drive the objective lens in the tracking direction and the focusing direction, tracking and focusing coils are provided on a lens holder which holds the objective lens. A magnetic circuit which constitutes electromagnetic driving circuits together with the tracking and focusing coils is provided on the carriage. The focusing and tracking coils are supplied with predetermined directions and intensities of electric current to drive the objective lens holder by the electromagnetic driving circuits, so that the laser beams can be accurately converged onto the recording track of the optical disc.
In the optical data recording and reproducing apparatus, it is necessary to provide a position sensor to detect the position (i.e., displacement from a correct position) of the objective lens (or objective lens holder), particularly in the tracking direction of the optical disc, to thereby reduce the track access time.
The position sensor usually includes a pair of reflecting surfaces formed on end surfaces of the objective lens holder in the tracking direction, and a pair of reflection type photo sensors on the carriage, each having a light emitter and a light receiver integral therewith, opposed to the corresponding reflecting surface. The outputs of the pair of photo sensors are identical at a neutral position of the objective lens holder in which the distances between the photo sensors and the corresponding reflecting surfaces are identical. When the objective lens holder is displaced from the neutral position, there is a difference in the output between the photo sensors, so that the displacement of the objective lens holder in the tracking direction can be detected in accordance with the output difference.
In the known position sensor, the reflecting surfaces are made of reflecting mirrors, which are separately constructed pieces secured to the end surfaces of the objective lens holder by an adhesive, for example. The adhering operation is, however, troublesome. In addition, it is difficult to precisely adhere both the reflecting mirrors to the corresponding end surfaces of the objective lens holder at correct positions, for example, due to a difference in position or thickness of the adhesive between the reflecting mirrors. This results in a failure to establish a precise optical location of the position sensor, leading to an incorrect detection thereby. To correct the error, an additional troublesome adjustment is necessary.
Moreover, in the known electromagnetic driving apparatus, the sensitivity at a low frequency band must be high, to obtain a high dynamic range. However, to realize this, the compliance of the objective lens holder support mechanism must be high so that the holder can be easily moved by a small driving force. However, a high compliance not only reduces the mechanical strength of the apparatus, but also lowers its resistance to shock. It is possible to support the objective lens holder on the carriage by strong springs in order to increase the mechanical strength. However, this would reduce the compliance to an inappropriate level.
The primary object of the present invention is to provide an electromagnetic objective lens driving apparatus in which reflecting surfaces can be precisely formed on an objective lens holder at correct positions.
Another object of the present invention is to provide an electromagnetic objective lens driving apparatus having an acceptable compliance and a high mechanical strength. In other words, the present invention is aimed at providing an increase in the sensitivity of the objective lens holder without reducing the mechanical strength thereof.